This ECG is from a 38 year old female with breast cancer and a pericardial effusion. She presented to the Emergency Department with shortness of breath and weakness. On physical examination, she was noted to have a 15 mm Hg fall in systolic blood pressure with inspiration and distended neck veins. The ECG shows sinus tachycardia with a rate of 120. There is diffuse elevation of the ST segment and depression of the PR segment, indicative of pericarditis. The QRS complexes are of low amplitude and there is QRS alternans, but it is less obvious than in the previous case. The echocardiogram revealed a large pericardial effusion and, as predicted by the QRS alternans, signs of cardiac tamponade. These were reversed by the removal of 400 ml of pericardial fluid.